The present invention relates to hydraulic devices used for forklifts, and, more particularly, to hydraulic devices for controlling tilt cylinders for tilting forklift masts.
A typical forklift includes a mast and a fork. The mast is supported by a vehicle body so that the mast tilts. The fork is supported by the mast so that the fork is lifted or lowered. The forklift also includes a spool type tilt cylinder and a spool type lift cylinder. The tilt cylinder tilts the mast forward or rearward with respect to the vehicle body. The lift cylinder lifts or lowers the fork. A valve unit is provided in the forklift for controlling the oil supply to these cylinders. The valve unit includes a tilt valve corresponding to the tilt cylinder and a lift valve corresponding to the lift cylinder. A tilt lever is arranged near the operator seat. By shifting the tilt lever, the opening of the tilt valve is varied so that the tilt cylinder tilts the most forward or rearward. In the same manner, a lift lever is arranged near the operator seat. By shifting the lift lever, the opening of the lift valve is varied so that the lift cylinder lifts or lowers the fork.
In accordance with the JIS (Japanese Industrial Standards), it is prescribed that, in principle, the maximum angle of the mast with respect to the vehicle body be six degrees when the mast tilts forward and twelve degrees when the mast tilts rearward. However, for example, if the mast is tilted forward by a large angle so that an object is transferred from the fork at a relatively high position, it is difficult to transfer the object in a stable manner. Thus, when tilting the mast forward while the fork carries an object at a relatively high position, it is preferable that the maximum mast angle be smaller than six degrees.
To solve the above problem, a device has been proposed for restricting the mast angle when the mast tilts forward when the fork is located at a relatively high position. The device includes a flow control valve such as a proportional electromagnetic type flow control valve or an electromagnetic type valve. The flow control valve is arranged in an oil passage connecting the tilt cylinder with the tilt valve. If the mast angle reaches a predetermined value (smaller than six degrees) when tilting forward, when the fork is located at a relatively high position, the control valve closes the oil passage so that the mast does not tilt further forward.
The tilt cylinder includes a first chamber and a second chamber, which are separated by a piston. When oil flows into the second chamber and flows out of the first chamber via the tilt valve, a piston rod of the tilt cylinder projects to tilt the mast forward. The flow control valve is arranged in the oil passage connecting the first chamber of the tilt cylinder with the tilt valve. When the control valve closes the oil passage, oil is not permitted to flow from the first chamber. The piston rod is thus not further projected, thus preventing the mast from tilting further forward.
The flow control valve is constituted by a spool typo valve like the tilt valve. The spool type valve includes a valve housing and a spool. The spool is arranged in the housing and slides between various positions. The oil passage is selectively opened or closed depending on the position of the spool. The opening of the oil passage is varied depending on the position of the spool. The outer surface of the spool contacts the inner surface of the housing so that the spool slides. A clearance, or a gap, exists between the outer surface of the spool and the inner surface of the housing The gap must be sealed to resist oil leakage. However, if the gap is very small or sealed so tightly that the oil does not leak, the sliding resistance of the spool against the housing increases. The spool then does not slide smoothly, and the opening of the valve does not vary quickly. To avoid this problem, the gap should be large enough to ensure smooth sliding of the spool. Therefore, a certain amount of oil actually leaks from the gap.
Thus, the flow control valve and the tilt valve permit some oil leakage from the hydraulic tilt cylinder circuit. This oil leakage may cause the mast to tilt when tilting should be halted. Specifically, the weight of the fork and the object on the fork act to tilt the mast forward, or to project the piston rod. Thus, oil is urged to flow from the first chamber. If the oil passage is closed by the flow control valve and the tilt valve to maintain the position of the mast, the oil pressure in the first chamber increases. Oil leakage occurs both from the flow control valve and the tilt valve through the gaps between the spools and the housings. Such oil leakage permits the piston rod to project, which permits the mast to tilt forward. Therefore, the mast fails to maintain its position, even when the oil passage is closed by the control valve and the tilt valve.